Core lock is a complication that can emerge in jet engines when they experience a flameout in flight. Flameouts occur when the flame inside the combustion chamber is extinguished, causing the engine to stop working. Pilots follow a precise set of procedures in an attempt to restart the engine. If they are unable to do so, core lock can develop, and the engine will experience a catastrophic failure. It may be possible to land the aircraft safely using the other engines and gliding techniques.

Jet engines are machined to extremely precise standards. Their moving parts are designed to work in a well-choreographed ballet when the engine is in operation to generate high speed and efficiency. When the engine abruptly stops working, the temperature change can be substantial. The metal components inside the engine are not designed for these conditions, and they can start to contract.

As parts contract, they may change shape and size at different rates. The narrow tolerances of the engine design may mean that parts start to interfere with each other and they freeze up. Inside the jet’s core, the components cannot rotate, which means that efforts like a windmill restart, where the aircraft dives to force the engine to turn again, will not be effective. Even if the flame in the combustion chamber can be relit, the components of the engine will not longer fit together, and it can’t restart.

Aircraft are designed with a number of failsafe technologies to prevent flameouts, reducing the risk that pilots will find themselves in a situation where they might experience core lock. In the event the engines do stop, pilots have a checklist of procedures to follow to get the engine started as quickly as possible. If this fails, they can discuss emergency landing options to see if it is possible to bring the plane to a safe stop at an airport or area of level ground. While the aircraft may be damaged, it could be possible to protect passengers and cargo.

This type of engine failure has been linked with several crashes. For core lock to occur, pilots usually need to make several mistakes in the handling of an emergency, and the ultimate cause of a crash may be considered pilot error. Jet engine designers routinely test and redesign their engines to address concerns like core lock, working to design equipment that is less likely to fail, even in extreme conditions.

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